Time synchronization is a critical piece of infrastructure for any distributed system. Distributed,
wireless sensor net-works make extensive use of synchronized time, but often have unique requirements in the
scope, lifetime and precision of the synchronization achieved, as well as the time and energy required to achieve
it. Existing time synchronization methods need to be extended to meet these new needs. We outline the
synchronization requirements of future sensor networks and present an implementation of our own low-power
synchronization scheme, post-facto synchronization. We also describe an experiment that characterizes its
performance for creating short-lived and localized but high-precision synchronization using very little energy.